Because Apple needs a new feature to sell the iPhone 6S
They could've easily added the feature to iPhone 6 by making it a bit thicker and increasing battery capacity, but they know that people will still buy the iphone 6 in spite of its missing features.
Sounds like an Apple answer...No need for a smaller device. The 6 + is bigger and its more prone to a camera shake.
Sounds like an Apple answer...
There's no reason why someone wouldn't want OIS. It helps to mitigate camera shake, something that is present with your arms out regardless of what they are holding.
Until you get into low light. Then OIS is the only way to go.